1. Field of the Invention
The present invention relates to a voltage controlled oscillator.
2. Description of the Related Art
A constitution of a conventional voltage controlled oscillator is shown in FIG. 5. A given bias voltage is applied to an oscillation transistor 31 by a series circuit not shown in the drawing. A resonance circuit 32 is constituted of a series circuit of a varactor diode 33 and a strip conductor 34. The strip conductor 34 is formed of two small strip conductors 34a, 34b which are connected in series. The varactor diode 33 has an anode thereof grounded through the strip conductor 34 and a cathode thereof is coupled to a base of the oscillation transistor 31 through a Clapp capacitor 35.
To a connection point between the small strip conductors 34a, 34b, an anode of a switching diode 36 is connected through a DC cut capacitor 37. A cathode of the switching diode 36 is grounded. Then, a cathode of the varactor diode 33 is connected to a control voltage terminal 39 through a choke inductor 38. To the control voltage terminal 39, a control voltage served for changing a capacitive value of the varactor diode 33 is applied. The anode of the switching diode 36 is connected to a band changeover terminal 41 through a resistance 40. A voltage is applied to the band changeover terminal 41 at the time of turning on the switching diode 36.
In such a constitution, when the switching diode 36 is turned on, both ends of the small strip conductor 34b are short-circuited by the switching diode 36 so that an oscillation frequency assumes a high band. On the other hand, when the switching diode 36 is turned off, the oscillation frequency assumes a low band. In both bands, the oscillation frequency can be changed in response to the control voltage.
In the above-mentioned prior art, although the oscillation is generated in a frequency range where a resonance circuit is inductive, the choke inductor is connected to the resonance circuit in parallel and hence, there has been a drawback that a change rate of an equivalent inductance value of the resonance circuit with respect to the change of capacity of the varactor diode is small and, particularly, sensitivity of change of the oscillation frequency in a low frequency band when the switching diode is turned off is lowered. FIG. 6 is a graph which shows a changing state of the oscillation frequency with respect to the control voltage applied to the varactor diode when the switching diode is turned off. From the graph, it is understood that the sensitivity is particularly low in a region where the control voltage is equal to or below 2.5 volts.
Further, in the above-mentioned conventional example, the switching diode has a capacitive component in an OFF state and, at the same time, a minute resistance component derived from a terminal is connected to the capacitive component in series. Eventually, this series circuit is connected to one small strip conductor in parallel. Due to such a constitution, and particularly due to the resistance component, a Q value of one strip conductor is lowered thus giving rise to a problem that C/N of oscillation signals is deteriorated.
Accordingly, it is an object of the present invention to provide a voltage controlled oscillator which can enhance the sensitivity of change of an oscillation frequency with respect to a control voltage applied to a varactor diode.
It is another object of the present invention to provide a voltage controlled oscillator which can prevent the deterioration of C/N of oscillation signals which may be caused by a minute resistance component of a switching diode.
To overcome the above-mentioned drawbacks of the prior art, according to the present invention, there is provided a voltage controlled oscillator which includes an oscillation transistor, a varactor diode, a resonance circuit which is coupled to the oscillation transistor, and a switching diode which changes over a resonance frequency of the resonance circuit, wherein the resonance circuit includes a first strip conductor which is served for grounding a cathode of the varactor diode in terms of direct current and a second strip conductor which is served for coupling a cathode of the varactor diode with the oscillation transistor, the switching diode is connected to the first strip conductor in parallel in terms of high frequency and has a cathode thereof grounded, and a control voltage which is served for changing a capacitive value of the varactor diode is applied to a connection point between the cathode of the varactor diode and the second strip conductor through a choke inductor.
Further, according to the present invention, there is provided a voltage controlled oscillator which includes a resonance circuit which is coupled to an oscillation transistor and is constituted of a series circuit of a varactor diode and a strip conductor, and a switching diode which is connected to one lengthwise portion of the strip conductor in parallel in terms of high frequency and changes over an oscillation frequency band in response to an ON state or an OFF state thereof, wherein an inductance element is connected to the switching diode in parallel in terms of high frequency, and an oscillation frequency when the switching diode is in the OFF state is substantially subjected to parallel resonance due to equivalent capacitive values of the inductance element and the switching diode in the OFF state.
Further, one end of the strip conductor and the cathode of the switching diode are grounded.
Still further, the strip conductor is divided into two small strip conductors, wherein one small strip conductor is connected between the varactor diode and the oscillation transistor, the other small strip conductor is connected between the varactor diode and a ground, and the switching diode is connected to the other small strip conductor in parallel.